Lip-rolling machines are used to produce a rolled rim on plastic containers. After trimming of the container from a thermoplastic sheet, the open end of the container has a rather sharp edge with an outward flare. The reason for rolling this edge is at least two-fold. First, drinking from a cup made in this fashion is a less than suitable undertaking for the user. Second, it is difficult to manufacture a lid which will fit about the flared edge and still maintain a suitable seal against the container rim.
Conventional lip rolling machines utilize an open ended, clam-shell type electric heating oven to heat and form lips on horizontally-nested thermoplastic containers, such as cups, as they are advanced off a production line. U.S. Pat. No. 6,093,010, issued Jul. 25, 2000, to Lamson, discloses a typical thermoforming process, including a clam shell oven. The oven often generates heat to cause internal surface temperatures of the oven to reach in excess of 500° F. with the heaters themselves getting up to 1200° F. However, when the production line is stopped—due, for example, to a thermoforming machine malfunction—a length of cups remain in the hot open oven. If this length of cups (approximately two feet of nested cups) is not removed from the oven when the line is stopped, these cups may catch fire or melt. Moreover, the line uses guide rails to stabilize the cups within the oven, but because of the extreme heat generated by the open-ended oven, it is necessary to run cooling water through the guide rails to keep the rails from overheating. This requirement adds to the expense and complexity of the machinery. Traditional lip rolling machines induce large amounts of heat into the open oven, and thus, utilize large amounts of energy.
Typically, lip rolling machines guide the cups along water rails up to a lip rolling screw assembly, within the clamshell oven, with the top open and the oven hot. Once the cups engage the screw(s), the oven lid is closed and the rim rolling process begins. As a result, a length of cups equal to or greater than the distance between a crowd roller assembly and the lip rolling screw assembly must be removed and discarded due to the insufficiency of heat applied to the cups from the oven being open and the lack of pressure applied by the crowd rollers.
The present invention is provided to solve these and other such problems with prior art devices.